Gaseous streams containing methane and ethane occur naturally, such as in natural gas and crude oil solution gas, and also as byproducts of a variety of refinery processes. In addition to methane and ethane, these gases often contain a substantial quantity of hydrocarbons of higher molecular weight, e.g., propane, butane, pentane and their unsaturated analogs.
Recent substantial increases in the market for the propane and heavier hydrocarbon components of natural gas have provided demand for processes yielding higher recovery levels of these products. Available processes for separating these materials include those based upon cooling and refrigeration of gas, oil absorption, refrigerated oil absorption, and the more recent cryogenic processes utilizing the principle of gas expansion through a mechanical device to produce power while simultaneously extracting heat from the system. Depending upon the pressure of the gas source, the richness (propane and heavier hydrocarbon content) of the gas and the desired end results, each of these prior art processes or a combination thereof may be employed.
Prior to the advent of the cryogenic expansion process, propane and the heavier component hydrocarbons were frequently separated by liquefaction and treatment with an absorption medium. The natural gas streams were contacted with an absorption oil (usually heptane), and the propane and the heavier hydrocarbon components were absorbed and thereafter desorbed and recovered.
In most present day refining processes, propane and the higher molecular weight components of natural gas and refinery gas are separated and recovered by liquefaction and cryogenic distillation at temperatures below 0.degree. F. Refrigeration for separation is supplied totally or partially by expansion of the gaseous stream in a turboexpander which produces power that may be used for example in driving a compressor.
In a typical cryogenic expansion-type recovery process, a feedstream gas under pressure is cooled by heat exchange with other streams of the process and/or external sources of cooling such as a propane compression refrigeration system. As the gas is cooled, liquids are condensed and are collected in one or more separators as a high pressure liquid feed containing most of the desired propane and heavier hydrocarbons. The high pressure liquid feed is transferred to a deethanizer column after its pressure is adjusted to the operating pressure of the deethanizer. The deethanizer is a fractionating column in which the liquid feed is fractionated to separate residual methane and ethane from the desired products of propane and heavier hydrocarbon components.
If the feedstream is not totally condensed (typically it is not), the vapor remaining from this partial condensation is expanded in a turboexpander to a lower pressure. Additional liquids are condensed as a result of the further cooling of the stream during expansion. The pressure after the expansion is usually the same pressure at which the deethanizer is operated. Liquids thus obtained are also supplied as a feed to the deethanizer. Typically, remaining vapor and deethanizer overhead vapor are combined as a residual methane/ethane product gas.
In the ideal operation of such a separation process, the vapors leaving the process will contain substantially all the methane and ethane found in the feed gas to the recovery plant and substantially no propane or heavier hydrocarbon components. The bottoms fraction leaving the deethanizer will contain substantially all the propane and heavier hydrocarbon components and essentially no methane or ethane. In practice, this ideal situation is not obtained because the conventional deethanizer is operated largely as a stripping column. Therefore, the methane and ethane vapors leaving the top fractionation stage of the column will contain vapors not subjected to any rectification step. Substantial losses of propane and heavier hydrocarbons occur because the vapors discharged from the low temperature separation steps contain propane and heavier hydrocarbon components which could be recovered if those vapors were brought to lower temperature, or if they were contacted with a significant quantity of a relatively heavy hydrocarbon, e.g. heptane, capable of absorbing the propane.
U.S. Pat. No. 4,272,269 which issued to Hammond, et al on June 6, 1981 describes one such process that combines both the cryogenic expansion step and the absorption process to increase the recovery percentage of the propane and hydrocarbon components. The disadvantage with using an absorption oil is that additional refining steps are needed to desorb the propane and prepare the absorption oil for reuse.
The problem associated with all types of propane recovery operations is one of efficiency. The main objective is to recover as much of the propane and heavier hydrocarbon components as is economically possible. The conventional systems in operation today are capable of economically recovering, at most, about 95% of the propane in a feedstream. Because of the large volume of gas that is processed, there is a definite need to find efficient methods to recover more of the propane and heavier hydrocarbons in a gaseous feedstream.